Two articles1,2 in this issue of JAMA address aspects of the well-established benefits of colorectal cancer screening. The first study, by Singh et al,1 attempts to estimate the effect of the interval between colonoscopic screening in an asymptomatic population on the benefits of the procedure. Although national guidelines recommend colonoscopy every 10 years,3- 5 this interval is not based on any direct, systematic evidence, but rather rests on anecdotal evidence and informed clinical judgment. The second study, by Lin et al,2 compares the findings of colonoscopy in an asymptomatic population 80 years and older with findings in a younger population aged 50 to 54 years. The practice of when to stop screening elderly individuals varies within the United States and throughout the world because little evidence on age-specific efficacy exists. These 2 studies address 2 very important and unresolved questions in colorectal cancer screening by colonoscopy, and the answers are made more difficult because the efficacy of colonoscopic screening has never been examined in a randomized trial.